Field of the Invention
The invention relates to a catalyst system and a recombination device for the recombination of hydrogen and/or carbon monoxide with oxygen, including a metallic carrier plate which is coated with a first and a second catalytic noble or precious metal, in particular for a nuclear power station. The invention also relates to a method for operating such a catalyst system and such a recombination device.
In a nuclear power station, after an incident with a loss of coolant, large quantities of hydrogen and carbon monoxide may be released into a safety vessel (containment). Without any countermeasure, it is possible for the atmosphere of the safety vessel to be enriched with hydrogen to such an extent that a detonatable mixture may be formed. In the event of late accidental ignition, the intactness of the safety vessel could be put at risk, above all due to the combustion of a relatively large quantity of hydrogen.
In view of those safety considerations, it is expedient to develop an early-starting recombination device in order to eliminate the hydrogen and carbon monoxide from the atmosphere of the safety vessel through the use of recombination with oxygen. That device should not lose activity appreciably, even in the event of a relatively long service life in the atmosphere of the safety vessel, and it should be capable of starting completely passively at low ambient temperatures. That is because the device is also intended to be used in the case of incidents at low ambient temperatures which may occur, for example, in TMI incident situations or by operating ventilation with resulting ambient temperatures of only 30.degree. C. or below.
The safety of a nuclear power station can be increased decisively through the use of such a recombination device, which must contain a number of specially constructed catalyst systems, since it is possible for the hydrogen to be reduced passively in good time.
European Patent EP 0 527 968 B1, corresponding to U.S. Pat. Nos. 5,473,646 and 5,301,217, discloses a recombination device, in which a number of catalyst systems in the form of plane plates, that are coated on both sides with catalyst material, such as platinum and/or palladium, are provided. That recombination device is particularly suitable for reducing hydrogen in the atmosphere of the safety vessel of a nuclear power station. In that case, each catalyst system includes a carrier plate, made of high-grade, stainless or special steel, which has on both sides a thin layer, the thickness of which is in the micrometer range and which is formed, in particular, of platinum and/or palladium. A multiplicity of such coated individual plates is disposed in a housing which may be constructed as a module. The gas stream to be monitored flows through the housing from below, and that gas stream leaves the housing in an upper region through an outlet orifice made laterally.
It must be stated that, in this case, each high-grade steel plate is provided completely with a monometallic coating on each side. That recombination device was provided, in particular, for use in serious incidents, in which temperatures of more than 50.degree. C. in the atmosphere of the safety vessel must be expected.
Furthermore, the prior art has described various noble or precious metal mixtures for the purpose of recombination (German Patent DE 36 04 416 C2, European patent Application EP 0 301 536 A2).
In order to avoid deactivation, the recombination devices known in the prior art may be additionally equipped with filters or else mounted in closed vessels which are opened if so required.
Such additional devices, although avoiding or reducing catalyst aging, are cost-intensive and complicated and can be constructed only with difficulty in the light of other requirements, such as, for example, earthquakes.